Microchannel heat exchanger module design to reduce water entrapment

ABSTRACT

A microchannel heat exchanger has a core having at least one heat exchange tube bank having a plurality of flow channels with a small hydraulic diameter less than 5 mm. A means is provided to reduce the amount of water retained on the external surfaces of the at least one heat exchange tube bank. These means may utilize the incorporation of a particular routing of refrigerant within the heat exchanger, the operation and control of a fan associated with the heat exchanger, or the provision of structure to at least partially block liquid from reaching the heat exchanger tube bank.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 61/095,019, which was filed Sep. 8, 2008.

BACKGROUND OF THE INVENTION

In recent years, much interest and design effort has been focused on efficient and durable operation of the heat exchangers in refrigerant systems. Sustained high effectiveness of refrigerant system heat exchangers directly translates into the augmented system performance and reduced life-time cost. One relatively recent advancement in heat exchanger technology is the development and application of parallel flow, or so-called microchannel or minichannel, heat exchangers (these two terms will be used interchangeably throughout the text), as the indoor and outdoor heat exchangers.

These parallel flow heat exchangers are provided with a plurality of parallel heat exchange tubes, typically of a non-round shape, among which refrigerant is distributed and flown in a parallel manner. The heat exchange tubes typically incorporate multiple channels and are orientated generally substantially perpendicular to a refrigerant flow direction in the inlet, intermediate and outlet manifolds that are in flow communication with the heat exchange tubes. Heat transfer enhancing fins are typically disposed in between and rigidly attached to the heat exchange tubes. The primary reasons for the employment of the parallel flow heat exchangers, which usually have all-aluminum furnace-brazed construction, are related to their superior performance, high degree of compactness, structural rigidity, reduced refrigerant charge and enhanced resistance to corrosion.

Microchannel heat exchangers provide beneficial results, at least in part, because their internal flow channels are of quite small hydraulic diameter. However, there are other challenges associated with microchannel heat exchangers. One challenge is that bare outdoor microchannel heat exchangers (as other heat exchanger types) are susceptible to atmospheric corrosion in industrial and coastal corrosive environments, due to the nature of their construction, material system and manufacturing processes.

In particular, the increased amount of water potentially retained on external heat exchanger surfaces and increased wet time, particularly in coastal corrosive environments, can present corrosion challenges.

Protective anti-corrosion coatings are known but are expensive. On the other hand, while less expensive coatings may be known, they are less effective. Therefore, it is desired to considerably reduce the amount of water retained on external surfaces of the outdoor heat exchanger (typically condenser or gas cooler), and thus significantly slow down corrosion reaction.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a microchannel heat exchanger is provided with at least one heat exchange tube bank having a plurality of flow channels with a hydraulic diameter less than 5 mm, and preferably less than 2 mm, and having a means incorporated into the heat exchanger and associated sub-system or structural design to reduce the amount of water retained on the heat exchanger external surfaces.

The means may utilize the incorporation of a particular routing of refrigerant within the heat exchanger, the operation and control of a fan associated with the heat exchanger, or the provision of structure to at least partially block liquid from reaching the heat exchanger tube bank.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a prior art arrangement of a microchannel heat exchanger.

FIG. 1B schematically shows one example of known heat exchanger.

FIG. 1C is a cross-sectional view through a tube bank.

FIG. 2 shows a first embodiment of the invention.

FIG. 3 shows a second embodiment of the invention.

FIG. 4 shows a third embodiment of the invention.

FIG. 5 shows yet another embodiment of the invention.

FIG. 6A shows another embodiment of the invention.

FIG. 6B shows a side view of a portion of the FIG. 6A embodiment.

FIG. 6C is a top view of a portion of the FIG. 6A embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical microchannel heat exchanger outdoor module 20 is illustrated in FIG. 1A. An upper deck 22 includes a fan system 24 for moving (typically pulling) air over a pair of microchannel heat exchangers 26 and 28. As can be appreciated, water accumulated inside outdoor module 20 will tend to collect near a lower portion 128 of this heat exchanger arrangement. Furthermore, moisture present in the atmospheric air, particularly in humid environments, will also accumulate on external heat exchanger surfaces. Due to the close-coupled construction of the microchannel heat exchanger, this moisture is retained within the heat exchanger core for prolonged periods of time. It should be noted that the outdoor module 20 shown in FIG. 1A is exemplary, and there many design variations of the outdoor module arrangements, including (but not limited to) vertical and V-shaped as well as straight and formed heat exchangers. All these designs and constructions are within the scope and can benefit from the invention.

As shown in FIG. 1B, the microchannel heat exchanger 26 includes an inlet 21 fluidly connected and delivering refrigerant to a top chamber 23 of an inlet/outlet manifold 28. After leaving the top chamber 23 of the manifold 28, refrigerant passes into a first heat exchange tube bank 25 and to a top chamber 27 of an opposed intermediate manifold 29. From the top chamber 27 of the manifold 29, the refrigerant returns through a second heat exchange tube bank 11 to an intermediate chamber 13 of the manifold 28. From the intermediate chamber 13 of the manifold 28, refrigerant passes through a third heat exchange tube bank 15 back to a bottom chamber 17 of the intermediate manifold 29. From the bottom chamber 17 of the manifold 29, the refrigerant passes through yet another forth heat exchange tube bank 19 to an outlet chamber 16 of the manifold 28. As shown, divider plates 43 divide manifolds 28 and 29 into the chambers 23, 13, 16 and 27, 17 respectively. In addition, fins 18 are positioned between the heat exchange tube banks 25, 11, 15, and 19. It should be noted that a four-pass heat exchanger configuration is exemplary, and different numbers of passes can be incorporated within the same heat exchanger construction. All these arrangements are within the scope of the invention.

As can be appreciated, in the condenser or gas cooler case, the hottest refrigerant (refrigerant typically leaving the compressor) is at the inlet 21 and within first heat exchange tube bank 25 of the heat exchanger 26, namely within the top section of the microchannel heat exchanger 26. As mentioned above, the greatest accumulation of water will be at the lower section of the microchannel heat exchanger 26. This top-to-bottom refrigerant flow arrangement is typical for microchannel condensers, since condensing refrigerant flow naturally coincides with the direction of gravity.

As shown in FIG. 1C, the heat exchange tubes of the tube banks include a plurality of small refrigerant channels 100 provided by separator walls 101. These channels have hydraulic diameter less than 5 mm, and preferably less than 2 mm. The channels can be any number of shapes and the term “diameter” does not imply a circular cross-section.

In FIG. 2, an embodiment 32 includes an inlet chamber 30 of an inlet/outlet manifold 180 at a vertically lower position leading to a heat exchange tube bank 40 passing refrigerant to a chamber 36 of an intermediate manifold 182. From the chamber 36, refrigerant passes through a heat exchange tube bank 42 to a chamber 31 of the inlet/outlet manifold 180, and back through yet another heat exchange tube bank 44 to another chamber 37 of the intermediate manifold 182. From the chamber 37, the refrigerant passes through a heat exchange tube bank 46 to an outlet chamber 33 of the inlet/outlet manifold 180. In the FIG. 2 embodiment, as opposed to the FIG. 1B prior art, the inlet chamber 30 is at a bottom section of the microchannel heat exchanger 32, providing a much hotter refrigerant to this section than would exist in the outlet chamber 33 at the heat exchanger exit.

By routing the hottest refrigerant into the inlet 30 positioned at the lower section of the microchannel heat exchanger, the hotter refrigerant will provide more heat to evaporate moisture retained on the external heat exchanger surfaces of the bottom area 128 of FIG. 1A, where the most amount of moisture is typically accumulated. Thus, the effect of corrosion at the most susceptible lower heat exchanger tube banks will be greatly reduced.

FIG. 3 shows an embodiment 60 wherein the inlet refrigerant line 61 is also at the vertically lowermost portion leading into an inlet chamber 62 of an inlet/outlet manifold 190. From the inlet chamber 62, the refrigerant passes through a heat exchange tube bank 64 to a chamber 66 in an intermediate manifold 192, a heat exchange tube bank 68, the intermediate chamber 67 of the inlet/outlet manifold 190, and through a branch refrigerant line 70 to another intermediate chamber 72 of the same inlet/outlet manifold 190 not adjacent to the chamber 67, leading in turn to a heat exchange tube bank 73. From the heat exchange tube bank 73, the refrigerant passes through yet another intermediate chamber 74 of the intermediate manifold 192, the heat exchange tube bank 76, and to the outlet refrigerant line 78. Essentially, this embodiment provides hotter refrigerant at the bottom and top heat exchanger tube bank sections 64 and 73, which might be more exposed to the effects of corrosion than the intermediate heat exchange tube banks 68 and 76. This may be beneficial, for instance, in situations when the top and bottom heat exchanger sections have reduced airflow and hence much lower water removal potential, in comparison to the center section. The FIG. 3 embodiment is purely exemplary, and other branch line configurations to provide intertwined refrigerant passes (in comparison to conventional staggered refrigerant passes) are also feasible and within the scope of the invention.

FIG. 4 shows an embodiment 80 wherein the refrigerant inlet line 82 is located within the top section of the microchannel heat exchanger. Refrigerant flow control devices such as valves 84 and 86 selectively route refrigerant through a tap line 88 to an injection point 90. If the valve 86 is open and the valve 84 is closed, refrigerant will pass normally into an inlet chamber 92 of inlet/outlet manifold 200, a heat exchange tube bank 94, an intermediate chamber 96 of an intermediate manifold 202, back through a heat exchange tube bank 98 to an intermediate chamber 112 of the inlet/outlet manifold 200. From the intermediate chamber 112 refrigerant passes through a heat exchange tube bank 103 to a chamber 105 of the intermediate manifold 202, and a heat exchange tube bank 102. From the heat exchange tube bank 102, the refrigerant passes through an outlet chamber 110 of the inlet/outlet manifold 200 and to an outlet refrigerant line 108. This embodiment will operate as in the prior art of FIG. 1B. However, either periodically, or when some indication has been received that there is moisture accumulating on the external surfaces of the lower heat exchange tube bank 102, the valve 86 may be closed or restricted and the valve 84 opened (partially or fully). Thus, at least a portion of hot refrigerant vapor from the inlet refrigerant line 82 will pass into the injection point 90. This hot refrigerant will provide additional heat to assist in evaporation of accumulated condensate on the external surfaces of the low heat exchange tube bank 102. The valves 84 and 86 can be ON/OFF solenoid valves or regulating valves and can be operated in a pulsation mode or in a modulation mode respectively. As was stated above, the FIG. 4 embodiment is exemplary, and other refrigerant bypass line configurations to provide a higher temperature refrigerant to the heat exchanger sections with increased amount of accumulated condensate on a periodic basis are also feasible and within the scope of the invention.

FIG. 5 shows yet another embodiment encapsulating a different way of reducing the condensate amount accumulated on external heat exchanger surfaces of an outdoor heat exchanger module 120. In the outdoor heat exchanger module 120, there are microchannel heat exchangers 122 and 124 and an air-moving device such as fan 126. The fan 126 typically operates in a forward direction to pull air over the microchannel heat exchangers 122 and 124 and then through a fan orifice 128. However, the fan 126 may be run in reverse to blow air over the heat exchangers 122 and 124, thus also blowing the accumulated condensate off of the external surfaces of the heat exchangers 122 and 124 (since airflow and gravity directions are coincidental now). The fan 126 may be run in reverse either periodically, or, again, when some indication has been received (such as, for instance, increased airside pressure drop) regarding condensate accumulation on external surfaces of the heat exchangers 122 and 124. Typically, axial fans provided within outdoor heat exchanger modules have sufficient airflow, while running in reverse, but other fan types can be utilized as well. If the fan 126 is a multi-speed or variable speed fan, then fan speed may be increased to reduce the condensate removal time. Further, if a multi-fan system is associated with the outdoor heat exchanger module 120, the number of operating fans may be increased to shorten the blow-off time as well. It should be pointed out that fan reversed operation can be coincidental with refrigerant system compressor operation, so that hot refrigerant circulating throughout the refrigerant system assists in condensate removal through evaporation, or fan reversals can be executed and controlled independently.

Furthermore, to remove condensate from external surfaces of the heat exchangers 122 and 124, during prolonged periods of shutdown in particular, fan system 126 can be turned on periodically, based on a timer or a sensor reading. Additionally, during normal operation, particularly at low ambient temperatures, a number of operational fans can be reduced (e.g. for a multi-fan system), or a speed of a variable speed fan can be reduced, to achieve lower airflow and higher temperature of the refrigerant circulating through the heat exchangers 122 and 124, thus resulting in faster condensate evaporation and heat exchanger dryout.

FIG. 6A shows an embodiment 129 intended to reduce the likelihood of rain water reaching the heat exchanger cores. Here, an upper deck 131 of the outdoor heat exchanger module 129 is generally solid. A fan orifice 133 receives a cap 134. Heat exchangers 130 thus are exposed to sufficiently reduced amount of water, since the cap 134 tends to divert the rain water radially outwardly and away from the heat exchangers 130. As shown in FIG. 6B, the cap 134 may be generally conical but other shapes or configurations (e.g. pyramidal) are also acceptable. FIG. 6C is a top view of the cap 134. Furthermore, the cap 134 may be formed of a wire mesh, perforated plate or the like, with sufficient porosity not to impede airflow provided by a fan 136 and small cell size preventing water to drain through the cap 134. On the other hand, the cap 134 may be made of solid material, and the airflow provided by the fan 136 will escape through the gap between the cap 134 and the upper deck 131.

The refrigerant systems that utilize this invention can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems. Also, although the invention is described in reference to microchannel heat exchangers and outdoor applications, such as condensers and gas coolers, it can be applicable to other heat exchanger types, such as round tube and plate fin heat exchangers, and indoor applications, such as reheat heat exchangers and evaporators. Furthermore, although the invention is described in reference to slanted heat exchanger configuration with horizontal tube orientation, it can be applied to vertical arrangements with either vertical or horizontal tube orientation.

Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A microchannel heat exchanger comprising: a heat exchanger core including at least one heat exchange tube bank with heat exchange tubes in the at least one heat exchange tube bank having a plurality of internal parallel flow channels; and a means to reduce condensate retention within the heat exchanger core.
 2. The heat exchanger as set forth in claim 1, wherein said means to reduce condensate retention within the heat exchanger core include routing of the refrigerant flowing inside said heat exchange tubes.
 3. The heat exchanger as set forth in claim 2, wherein there are a plurality of said heat exchange tube banks, and refrigerant flows in opposed parallel directions through said plurality of heat exchange tube banks from an inlet manifold, into an intermediate manifold, and from said intermediate manifold to an outlet manifold, said inlet manifold fluidly connected to a first heat exchange tube bank, with both said inlet manifold and said first heat exchange tube bank being located in a bottom section of the heat exchanger to provide a higher temperature refrigerant to the bottom section of the heat exchanger.
 4. The heat exchanger as set forth in claim 3, wherein there are more than two of said heat exchange tube banks, with said first heat exchange tube bank being a vertically lowermost of said heat exchange tube banks, and one of intermediate heat exchange tube banks being a vertically uppermost of said heat exchange tube banks.
 5. The heat exchanger as set forth in claim 3, wherein there are more than two of said heat exchange tube banks, with said heat exchange tube banks being arranged in a vertically intertwined configuration.
 6. The heat exchanger as set forth in claim 5, wherein at least one branch pipe routes refrigerant from one of said heat exchange tube banks to another of said heat exchange tube banks.
 7. The heat exchanger as set forth in claim 3, wherein a flow control device for selectively tapping at least a portion of higher temperature refrigerant from an upstream location to a downstream location to provide additional heating at the downstream location is included.
 8. The heat exchanger as set forth in claim 7, wherein said downstream location is in an intermediate manifold.
 9. The heat exchanger as set forth in claim 8, wherein said intermediate manifold communicates with a vertically lowermost one of said plurality of heat exchange tube banks.
 10. The heat exchanger as set forth in claim 1, wherein there is at least one fan associated with the heat exchanger, said at least one fan being operable to move air over said at least one heat exchange tube bank to absorb heat from refrigerant flowing inside said heat exchange tubes, said at least one fan being selectively operable in a reverse direction to move air over said at least one heat exchange tube bank to remove moisture accumulated on external heat exchanger surfaces.
 11. The heat exchanger as set forth in claim 1, wherein there is at least one fan associated with the heat exchanger, said at least one fan being operable to move air over said at least one heat exchange tube bank to absorb heat from refrigerant flowing inside said heat exchange tubes, said at least one fan is a variable speed fan and said variable speed fan is selectively and periodically operated at a reduced speed to increase temperature of refrigerant flowing through at least one heat exchange tube bank to remove moisture accumulated on external heat exchanger surfaces.
 12. The heat exchanger as set forth in claim 1, wherein there is at least one fan associated with the heat exchanger, said at least one fan being operable to pull air over said at least one heat exchange tube bank to absorb heat from refrigerant flowing inside said heat exchange tubes, said at least one fan to be periodically turned on during prolonged shutdown periods to move air over said at least one heat exchange tube bank to remove moisture accumulated on external heat exchanger surfaces.
 13. The heat exchanger as set forth in claim 1, wherein there are at least two fans associated with the heat exchanger, said at least two fans being operable to move air over said at least one heat exchange tube bank to absorb heat from refrigerant flowing inside said heat exchange tubes, and at least one fan of said at least two fans to be selectively and periodically turned off to increase temperature of refrigerant flowing through at least one heat exchange tube bank to remove moisture accumulated on external heat exchanger surfaces.
 14. The heat exchanger as set forth in claim 1, wherein there is a frame structure associated with the heat exchanger, said frame structure including a generally solid upper deck and a fan mounted to said frame structure and moving air over said at least one heat exchange tube bank through a fan orifice, and a cover for blocking moisture from entering said fan orifice.
 15. The heat exchanger as set forth in claim 14, wherein said cover includes a wire mesh material. 